Silicon nitride circuit substrate and semiconductor device containing same

1. A silicon nitride circuit substrate, comprising: a silicon nitride substrate; a metal circuit plate; and an intermediate layer being interposed between the silicon nitride substrate and the metal circuit plate for joining the silicon nitride substrate and the metal circuit plate, and having a compound containing an aluminum oxide component, in which the concentration of the aluminum oxide component in the intermediate layer is higher in the side of the metal circuit plate than in the side of the silicon nitride substrate, and wherein the concentration of the aluminum oxide component in the side of the metal circuit plate is 40% by weight or more.

2. The silicon nitride circuit substrate of claim 1, wherein the thickness of the intermediate layer is within a range of 0.5 to 15 .mu.m.

3. The silicon nitride circuit substrate of claim 1, wherein the compound containing an aluminum oxide component includes a substance which is selected from the group consisting of alumina, aluminosilicate and rare earth aluminate, and the aluminosilicate includes mullite, and the aluminum oxide component forms a M-Al-O compound wherein the M represents a metal component composing the metal circuit plate.

4. The silicon nitride circuit substrate of claim 1, wherein the intermediate layer contains a compound containing a titanium component so that the concentration of the titanium component in the intermediate layer is higher in the side of the silicon nitride substrate than in the side of the metal circuit plate.

5. The silicon nitride circuit substrate of claim 4, wherein the titanium component of the intermediate layer forms a titanium-aluminum compound or a titanium-silicon compond, in which the titanium-aluminum compound includes TiAl.sub.3 and Ti.sub.2 AlN and the titanium-silicon compound cinludes Ti.sub.5 Si.sub.3.

6. The silicon nitride circuit substrate of claim 1, wherein the intermediate layer contains a nitrogen component in such a manner that the concentration of the nitrogen component in the intermediate layer is higher in the side of the silicon nitride substrate than in the side of the metal circuit plate.

7. The silicon nitride circuit substrate of claim 1, wherein the silicon nitride substrate has a grain boundary phase such that 20% by volume or more of the grain boundary phase is a crystal phase, and the silicon nitride substrate has a porosity of 1.5% by volume or less, a heat conductivity of 60 W/mK or more at 25.degree. C. and a three-point bending strength of 60 kgf/mm.sup.2 or more.

8. The silicon nitride circuit substrate of claim 1, wherein the silicon nitride substrate contains a rare earth element in an amount of 1.0 to 12.5% by weight converted into an oxide, and an impurity positive ion elements which includes Li, Na, K, Fe, Ca, Mg, Sr, Ba, Mn and B is contained in the silicon nitride substrate in a total amount of less than 0.3% by weight.

9. A semiconductor device, comprising: a silicon nitride circuit substrate, comprising: a silicon nitride substrate; a metal circuit plate; and an intermediate layer being interposed between the silicon nitride substrate and the metal circuit plate for bonding the silicon nitride substrate and the metal circuit plate, and having a compound containing an aluminum oxide component, in which the concentration of the aluminum oxide component in the intermediate layer is higher in the side of the metal circuit plate than in the side of the silicon nitride substrate, and wherein the concentration of the aluminum oxide component in the side of the metal circuit plate is 40% by weight or more; and a semiconductor element which is mounted on the silicon nitride circuit substrate.